We are studying the mutational process in Mammalian somatic cells with special emphasis on the role of alterations in regulatory control factors as a source of cell variation. For this we are using a Chinese hamster ovary (CHO) cell system we have developed capable of distinguishing between regulational and structural gene mutational events and a human choriocarcinoma cell line (JEG) regulationally variant for a selectable enzyme. The exposure of CHO cells to ultraviolet radiation, nonselective isolation of single cell clones and the examination of the clones for alterations in the electrophoretic mobility and expression of the products of over 40 enzyme loci proved an effective method of detecting mutations in Mammalian cells. Mutants were indentfied with respect to alterations in specific proteins and fell into four classes which included selective systems, structural gene as well as possible control factor modifications. Unlike results from selective systems, structural gene electrophoretic mutants revealed CHO cells to be functionally and chromosomally quasi-diploid and dose response studies indicated that the frequency of mutants/survivor peaked instead of being linear with dose. Use of a selective system (6-thioguanine resistance) followed by screening survivors through the enzyme battery after electrophoresis resulted in double mutants. This double mutant technique will now be used to measure the frequecy of electrophoretic mutants at two different UV dose levels to determine not only the effectiveness of enrichment for variants, but also the kinetics of drug resistant and electrophoretic mutants in the same cells. Effects of chemical mutagens on the system will be begun. Somatic cell hybridization analysis will be begun to study the interraction and segregation of the control as well as structural gene mutants. Through electrophoretic, chromosomal and somatic cell hybridization analysis we have determined adenosine deaminase (ADA) to be under negative regulational control in the human JEG cells. We shall analyze the genetics of that control in human-mouse hybrid clones aad devise a system (through) the use of ara-adenosine) to select for ADA "turn on" in these cells. This system will then be evaluated as a screen for detecting agents which alter regulatory states in human cells.